1. Field of the Invention
The present invention relates to a muffler of a car or a motorbike, and in particular to a muffler made of a titanium alloy wherein advantages of lightness and corrosion-resistance that the titanium alloy originally has are used, and heat-resistance and oxidization-resistance are heightened without damaging costs or workability so that the span of life and flexibility for design are improved.
2. Description of Related Art
An exhaust system of a car or a motorbike is composed of an exhaust manifold, an exhaust pipe, a catalyst muffler, a pre-muffler, and a silencer (main muffler), which are, in this order, arranged from the exhaust gas outlet side of the engine. In the present specification, the generic term xe2x80x9cmufflerxe2x80x9d is given to any one of these members or the whole thereof. As a constituent material of the muffler, ordinary steel was used in old times. In recent years, stainless steel superior in corrosion-resistance has been mainly used.
Concerning some fields, mainly the field of motorbikes, attention has been paid to a muffler made of Ti in more recent years. Cases in which each of standard grade motorbikes that are mass-produced, including motorbikes for a race, is equipped with a muffler made of Ti have been increasing since Ti, which is different from ordinary steel or stainless steel in the prior art, has the following characteristics 1)-4).
1) Ti has a specific gravity of about 60% of steel-type material. Thus, Ti is very light so that cars or motorbikes can be made light.
2) Ti has very good resistance against corrosive gas or corrosive liquid containing salt and exhaust components. Thus, problems about corrosion are wholly overcome. (Even stainless steel, which is generally said to have superior corrosion-resistance, undergoes corrosion based on salt scattered on the road surface to prevent freezing of the surface in winter.)
3) Since Ti is light, load stress based on vibration at the time of driving an engine is reduced. Thus, durable resistance against vibration fatigue is improved.
4) Ti has a smaller thermal expansion coefficient than steel. The thermal expansion coefficient is about 70% of that of ordinary steel, and about 50% of that of stainless steel. Therefore, stress load associated with thermal expansion is small. Thus, durable resistance against thermal fatigue is also superior.
In almost all of mufflers made of Ti which are made practicable at the present time, pure titanium of the JIS second grade, for industrial use, is used. It is predicted that the temperature of exhaust gas from cars or motorbikes is usually about 700xc2x0 C. or higher. However, in the case that the outer surface of a muffler is large and is open to the air outside, as in motorbikes, heat radiates from the surface to the open air. Thus, the temperature of the muffler itself does not rise very much. Even the pure titanium of the JIS second grade can be used without any trouble. However, the temperature of metal positioned in an exhaust pipe in car mufflers, which is not directly open to the air, or metal positioned at a part where exhaust pipes joint in mufflers for motorbikes rises easily to a high temperature. Therefore, a Ti alloy having a higher heat-resistance than the JIS second pure titanium is desired. In the case that a Ti alloy having high heat-resistance and high strength is used, the Ti alloy positioned in sites whose temperature rises within a cold temperature range (a low temperature range of room temperature to about 400xc2x0 C.) can also be made thin. Accordingly, it can be expected that the muffler can be made still lighter than JIS second grade pure titanium and the flexibility of design can be improved.
From such viewpoints, it can be considered that Ti alloys such as Ti-3Al-2.5V and Ti-6Al-4V, among existing titanium alloys, are hopeful materials for mufflers. However, for forming and fabrication into a muffler, a raw material needs to be made thin and must have superior workability. The above-mentioned two existing Ti alloys, which are insufficient in forming-workability, cannot satisfy the requirements.
Specifically, the above-mentioned Ti-6Al-4V is unsuitable for a material for mufflers such as an exhaust pipe and a silencer since this alloy cannot be worked into a thin plate by cold rolling. On the other hand, Ti-3Al-2.5V can be considered as the most hopeful material for mufflers among existing titanium alloys since this alloy can be cold-rolled to some extent and worked into a thin plate. In this titanium alloy, however, a border crack or an internal defect is easily generated in a cold rolling step. Thus, it is necessary that rolling and intermediate annealing are repeated plural times. As a result, costs for working to a thin plate are very high. Moreover, this alloy is far poorer in workability at the time of secondary working to a muffler than JIS second grade pure Ti materials.
In light of the above-mentioned situations, the present invention has been made. An object of the present invention is to provide a muffler superior in heat-resistance and oxidation-resistance, using a Ti alloy having the following performances.
1) The Ti alloy has better heat-resistance and oxidation-resistance than JIS second grade pure Ti materials, and can be applied to high temperature sites of a muffler.
2) Cold workability, which is insufficient in conventional Ti alloys having superior heat-resistance (Ti-3Al-2.5V and Ti-6Al-4V), is improved. Cold workability to a thin plate and workability to a muffler are made as high as JIS second pure Ti materials.
3) The Ti alloy is an alloy that can keep superior weldability since joint based on welding is essential in working to a muffler.
The muffler, made of a titanium alloy, of present invention that has attained the above-mentioned object is a muffler made of a titanium alloy, wherein the titanium alloy comprises 0.5-2.3% by mass of Al. By using this titanium alloy, it is possible to keep heat-resistance and oxidation-resistance required for a muffler and improve forming-workability. Therefore, a muffler that is suitable for production for working into a tube form and is thinner and lighter can be realized by curving a cold-rolled plate of the present titanium alloy and then subjecting the plate to seam welding.
Preferably, the titanium alloy is a binary-element alloy comprising Ti-(0.5-2.3%)Al. Any alloying element other than Al may be incorporated so far as the feature of the present invention is not lost. In this case, in order to keep heat-resistance and oxidation-resistance and improve workability sufficiently, it is preferred that the ratio of the xcex1 phase in metal texture of the titanium alloy is over 90% or more by volume.
The xe2x80x9cmufflerxe2x80x9d referred to in the present invention is a generic term given to any one member of an exhaust manifold, an exhaust pipe, a catalyst muffler, a pre-muffler, a silence (main muffler) and the like, or the whole thereof. In other words, the xe2x80x9cmufflerxe2x80x9d in the present invention means whole or a part of an exhaust system. The xe2x80x9cmufflerxe2x80x9d in the present invention can be applied not only to a car or a motorbike but also to a ship or other machinery.